The message
from Owain contains these words:

You don't fit them or any other electrical accessory "in amongst" the
tiles - you take the accessory off the wall and continue the tile a few
mm underneath (inside) the perimiter of the accessory, then put the
accessory back on the surface of the tile.

And doesn't it look vile when some lazy bugger has retiled over the top
of old tiles (which is bad enough on its own) and not bothered to lift
the sockets and switches out. Uuuurghlenarsty.

--
Skipweasel
Pay no attention to that man behind the curtain.